1. Field of the Invention
This invention relates to optical fiber scintillators for use in detecting x-rays, .gamma.-rays, thermal neutrons, and the like. More particularly, this invention relates to improved, high efficiency, high spatial resolution optical fiber arrays for use as scintillator plates in radiographic imaging systems and particularly real-time imaging systems.
2. Description of Related Art
Conventional radiographic imaging systems which are used medically for diagnosis and industrially for inspection, typically contain a detector which is a combination of an inorganic phosphor screen with a photographic film or with an image intensifier, photo-electric detector. When it is necessary to view the image instantaneously, i.e., in real-time, the image intensifier, photo-electric detector is used in combination with suitable monitor system. Such inorganic phosphor screens which are used extensively for high energy photon imaging suffer from the compromise between detection efficiency and spatial resolution. This compromise is the consequence of the low interaction probability of the high energy photon within the phosphor screen thickness to produce scintillation light. When the interaction probability is increased by increasing the thickness of the screen, spatial resolution of the incident high energy photons suffers since the isotropically produced scintillation light is spread over a larger area. Loss in spatial resolution is further exacerbated by light scatter due to the granularity of such thick phosphor screens. Thus, screens are manufactured to be as thin as possible without unduly limiting sensitivity.
In an attempt to overcome the inherent limitations of conventional phosphor screens, fiber optic scintillator plates have been devised for real time imaging systems. Such a plate is described in Brown, U.S. Pat. No. 4,415,810 and in the publication "High-Density Glass Scintillator for Real-Time X-Ray Inspection" by Placious et al. in MATERIALS EVALUATION, Vol. 49, No. 11, pp. 1419-21, November, 1991. Brown discloses a device for imaging penetrating radiation which includes a multiplicity of contiguously disposed fiberoptic tubes each of which is made of a very small diameter with an inner core material composed of scintillating material and a cladding glass about the inner core to serve as a reflector and a third and outer layer which is light absorbing to absorb stray light from the inner core. The specific scintillating material disclosed is a glass formulation containing lithium and cesium. Brown discloses that the device may be used in the electron image detection system of U.S. Pat. No. 4,096,381. Placious et al. disclose a formulation for terbium-activated radioluminescent glass which produced a glass scintillator with excellent x-ray imaging detection characteristics.
Two studies have been published relating to the use of scintillating optical fiber arrays for x-ray imaging. Scintillating optical fiber arrays for high resolution x-ray imaging over 5 keV was disclosed by E. Bigler and F. Polack in APPLIED OPTICS; Vol. 24, No. 7; pp. 994-997; 1 Apr. 1985. Bigler et al. disclose an x-ray image detector having a 10 - 5 .mu.m resolution for 5 keV x-rays in which the detector consists of an array of optical fibers, the core of which has been replaced by a high-index fluorescent material. In practice, the optical fibers were an array of microtubes filled with a liquid scintillator, i.e., a benzene or toluene solution of an organic fluorescent material. Other scintillating materials suggested for investigation included glasses, plastics and gels. A study of the physics of scintillating fiber optics and their applications in radiographic systems was disclosed by H. Shao, D. W. Miller and C. R. Pearsall in IEEE TRANSACTIONS ON NUCLEAR SCIENCE; Vol. 38, No. 2; pp. 845-857; April, 1991. In their study, Shao et al. used Tb.sub.2 O.sub.3 -based scintillating fiber optic arrays and concluded that scintillating fiber optic arrays are superior to the phosphor screen at all x-ray energies except low energy (i.e., below 90 keV).
Organic plastic scintillators have been used for scintillation counting of high energy particles and radiation and is described in Chapter 9 of THE THEORY AND PRACTICE 0F SCINTILLATION COUNTING by J. B. Birks, 1964, Pergamon Press, Oxford. In particular, on pages 339 and 340, Birks reviews various attempts to incorporate compounds containing heavy elements into plastic scintillators, in order to increase .gamma.-ray absorption coefficient, without causing too great a deterioration in scintillation efficiency due to impurity quenching. Illustrative of organic plastic scintillators which are free of heavy metal elements, are the plastic scintillators disclosed in Shimizu et al., U.S. Pat. No. 4,495,084 and the high temperature plastic scintillators of Simonetti, U.S. Pat. No. 4,713,198.
Quench-resistant fluors for liquid scintillation counting have been disclosed by S. W. Wunderly and J. M. Kauffman in APPL. RADIAT. ISOT.; Vol. 41, No. 9; pp. 809-815; 1990; INT. J. RADIAT. APPL. INSTRUM.; Part A. Wunderly et al. disclose fluors which are diaryl derivatives of fluorene or 2,2'-bifluorene and which were considered to be the most efficient fluor systems studied, particularly when chemically quenched in a non-aqueous system, or quenched with water in an emulsifier-containing system.
Radiation sensitive optical fibers and their use as detectors are disclosed in Koechner, U.S. Pat. No. 4,788,436. Koechner discloses an optical fiber sensitive to nuclear radiation which includes a thin filament core formed of a plastic scintillator material which scintillates in the presence of nuclear radiation, and an optical cladding material around the optical fiber. The specific scintillator materials disclosed by Koechner is polyvinyltoluene or polystyrene which are doped with organic scintillator material.
While scintillator plates of fiber optic arrays have improved the resolution of real-time radiographic systems, there continues to be a need to improve the efficiency of the plate in detecting x-rays, .gamma.-rays and thermal neutrons, so as to at least equal the efficiency of screen/film systems without compromise of resolution. This need is particularly important in the area of medical diagnostics and treatment wherein exposure of the patient to high energy radiation is to be minimized. There is an additional need for a fiber optic plate with improved efficiency in exclusively detecting thermal neutrons without detecting spurious .gamma.-rays or x-rays.